calculating duration between two cells based on matching criteria

Bit of a complex one...

I have a time/date column called [Date Created] and a [Username] column, I want to calculate the duration between the first and next times - when the [Username] is the same, and the date of the [Date Created] field is the same. So my new column called [New Start Time] should populate the next [Date Created] against each row when the user is the same, and the date is the same, else it is left blank.

Before I created my calculated column, I sorted the [Date Created] ascending and added an [Index] column. Because I want it to check the username AND the date are the same, I also added a [Weekday] column.

My calculated column is below, and it works inconsistently which means sometimes it's populating the next [Date Created] when it's a different day, thus the duration between is 3,000 minutes instead of about 20 minutes. And sometimes it ignores the rule and leaves a blank [New Start Time] when the username and day is the same. So, I think my measure is wrong, and i'm hoping someone can help me!!

New Start Time = IF(AND([Username] = LOOKUPVALUE([Username],[Index],[Index]+1), LOOKUPVALUE([Weekday],[Index],[Index]+1)),
LOOKUPVALUE([Date Created],[Index],[Index]+1),BLANK())